1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device that may improve image quality by reducing a leakage current in a backlight and a fabrication method thereof.
2. Description of the Background Art
In the case of a display device, especially a flat panel display such as a liquid crystal display device, active devices such as thin film transistors (TFT) are formed in each pixel to drive the display device. Normally, this driving method of the display device is called an active matrix driving method. In the active driving method, the active devices are disposed in the pixels aligned in a matrix respectively, for driving the pixels.
FIG. 1 is a plan diagram illustrating an active matrix type liquid crystal display device. Referring to FIG. 1, the liquid crystal display device is a TFT LCD using TFTs as active devices. N×N pixels are vertically and horizontally disposed in the TFT LCD. In each pixel of the TFT LCD, the TFT is formed at the crossing of a gate line 4 receiving a scan signal from an external driving circuit and a data line 6 receiving an image signal. The TFT includes a gate electrode 3 connected to the gate line 4, a semiconductor layer 8 formed on the gate electrode 3 and activated when the scan signal is applied to the gate electrode 3, and source/drain electrodes 5a and 5b formed on the semiconductor layer 8. A pixel electrode 10 is connected to the source/drain electrodes 5a and 5b receives the image signal through the source/drain electrodes 5a and 5b by activation of the semiconductor layer 8, and drives liquid crystals (not shown) that are formed in the display region of the pixel 1.
FIG. 2 is a cross-sectional diagram illustrating the structure of the TFT disposed in each pixel. As shown in FIG. 2, the TFT includes the gate electrode 3 formed on a substrate 15 made of a transparent insulator such as glass, a gate insulating layer 11 stacked on the whole surface of the substrate 15 on which the gate electrode 3 has been formed, the semiconductor layer 8 formed on the gate insulating layer 11 and activated when a signal is applied to the gate electrode 3, the source/drain electrodes 5a and 5b formed on the semiconductor layer 8, and a passivation layer 13 formed on the source/drain electrodes 5a and 5b, for protecting the device. Here, the drain electrode 5b of the TFT is electrically connected to the pixel electrode 10 formed in the pixel. Therefore, when the signal is applied to the pixel electrode 10 through the source/drain electrodes 5a and 5b, the pixel electrode 10 drives the liquid crystals to display images.
On the other hand, in a 4-mask process, the semiconductor layer 8 and the source/drain electrodes 5a and 5b are formed by the same mask process. The source/drain electrodes 5a and 5b are formed to correspond to the semiconductor layer 8.
However, in the related TFT structure, light (indicated by arrows) of a backlight is incident on the semiconductor layer 8 and may activate the semiconductor layer 8 that increases a leakage current. As a result, the related art liquid crystal display device has reduced image quality by generating an afterimage on the screen due to the increased leakage current caused by light of the backlight.